Antenna and wireless tag

Antenna and wireless tag

An antenna (1) in accordance with the present invention includes a ground plate (11) and an antenna element (12) provided on an identical plane or on different planes parallel to each other, and is suitable for use in a wireless tag (2). The antenna (1) in accordance with the present invention further includes a shortening capacitor (14) that bridges the ground plate (11) and an end part (12B) of the antenna element (12), the end part (12B) being opposite to the power feed-side end part.Related Terms:AntennaCapacitorWirelessWireless Tag

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of PCT International Application No. PCT/JP2012/068504 filed in Japan on Jul. 20, 2012, which claims the benefit of Patent Application No. 2011-159216 filed in Japan on Jul. 20, 2011, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

BACKGROUND ART

In recent years, an RFID (Radio Frequency Identification) system has been widely used for various purposes. The RFID system includes a wireless tag and a reader, and performs various functions via wireless communications between the tag and the reader.

The wireless tag for use in the RFID system is categorized into a passive tag that contains no battery and an active tag that contains a battery. The passive tag is used as a wireless tag for wireless communications between itself and a reader that is close to it (e.g., for use as a pre-paid card). On the other hand, the active tag is used as a wireless tag for wireless communications between itself and a reader that is not close to it (e.g., for use as a tag carried by a user in a presence management system or as a tag attached to a commercial product in an inventory management system). The presence management system is disclosed in, for example, Patent Literature 1.

It is necessary that the wireless tag include an antenna for wireless communications between itself and a reader. As the antenna included in the wireless tag, a small loop antenna is often used regardless of whether the wireless tag is an active tag or a passive tag. However, the active tag including a small loop antenna has too small a radiative power that it may cause a problem in wireless communications between the tag and a reader that is not close to the tag.

One way to solve such a problem is to use a small dipole antenna or a small monopole antenna instead of the small loop antenna. It should be noted here that the small dipole antenna and the small monopole antenna mean a dipole antenna and a monopole antenna, respectively, each having an antenna element whose total length ρ is much smaller than its resonant wavelength λ (i.e., ρ<<λ). The radiative power of the small loop antenna is proportional to (ρ/λ)4, whereas the radiative power of each of the small dipole and monopole antennas is proportional to (ρ/λ)2. That is, the radiative power of each of the small dipole and monopole antennas is greater than the radiative power of the small loop antenna. However, both of these small antennas, which satisfy ρ<<λ, can only achieve a limited level of radiative power.

On the other hand, a half-wave dipole antenna satisfying ρ=λ/2 and a quarter-wave monopole antenna satisfying ρ=λ/4 are known to have better radiation efficiencies than the above-mentioned small antennas. An active tag including a half-wave dipole antenna is, for example, one that is disclosed in Patent Literature 2. The active tag disclosed in Patent Literature 2 includes a planar half-wave dipole antenna and thereby achieves a sufficient radiative power and also achieves a small thickness.

SUMMARY

In order for a wireless tag to comply with laws and regulations such as the Radio Act, it is often necessary that an antenna included in the tag operate in a low frequency band (low resonant frequency). However, if a card-shaped wireless tag including a planar half-wave dipole antenna or a quarter-wave monopole antenna is designed to operate in a lower frequency band, the antenna should become larger. Therefore, a card-shaped wireless tag including a planar half-wave dipole antenna or a planar quarter-wave monopole antenna cannot meet a demand for downsizing the wireless tag.

For example, in Japan, the maximum electric field strength allowed for low power radio stations such as a wireless tag is specified as shown in FIG. 13 (refer to Article 4 of the Radio Act and Article 6 of Regulations for Enforcement of the Radio Act). Specifically, in a frequency band of 322 MHz and lower, the use of a low power radio station without a license is permitted provided that the electric field strength (technically, the electric field strength at 3 meters from the low power radio station) is 500 μV/m or less; on the other hand, in a frequency band of not lower than 322 MHz but not higher than 10 GHz, the use of an extremely low power station is not permitted without a license if the electric field strength is greater than 35 μV/m. If the electric field strength is reduced to equal or less than 35 μV/m, at worst, an electromagnetic wave may not be strong enough even at a distance of 1 meter from the low power radio station. Such a low power radio station is not practical. In a frequency band of higher than 10 GHz, the electric field strength greater than 35 μV/m is permitted. However, as the frequency increases, it becomes more difficult to produce parts that constitute the low power radio station. In particular, parts for a low power radio station that operates in a frequency band of 60 GHz and higher have not been put in practical use as of now. Therefore, in order to realize a wireless tag that is easily accessible to everyone, it is preferable that an antenna included in the wireless tag has an operating frequency band of 322 MHz and lower.

However, in order to realize a half-wave dipole antenna designed to operate in a frequency band of 322 MHz and lower, it is necessary that the total length of an antenna element of the half-wave dipole antenna be λ/2≈46.6 cm or greater. Therefore, given that the half-wave dipole antenna is a planar antenna, it is difficult to meet the demand of reducing the size of the wireless tag to, for example, 85.6 mm×54.0 mm. Furthermore, in order to realize a quarter-wave monopole antenna designed to operate in a frequency band of 322 MHz and lower, it is necessary that the total length of an antenna element of the quarter-wave dipole antenna be λ/4≈23.3 cm or greater. Therefore, even with a planar quarter-wave dipole antenna, it is difficult to meet the above demand. This problem arises regardless of whether or not the antenna element and a ground plate are provided on the same plane.

It should be noted that the above-mentioned size, i.e., 85.6 mm×54.0 mm (more technically, 85.60 mm×53.98 mm), is the size of a card specified as ID-1 in ISO/IEC7810. This size is often used for a passive tag such as an e-cash card. The size of an ID-1 card has a golden aspect ratio. Therefore, it looks good and also is internationally recognized as the size of a card that can be easily handled by humans. If it was possible to realize an active tag having the size of an ID-1 card, that would be ideal. However, as described earlier, it is difficult to realize an active tag having the size of an ID-1 card with the use of an existing half-wave dipole antenna or quarter-wave monopole antenna.

The present invention has been made in view of the above problems, and an object of the present invention is to realize an antenna which has a radiation efficiency as high as that of a quarter-wave monopole antenna, the antenna being designed to operate in a lower frequency band without increasing its size. In particular, an object of the present invention is to realize an antenna that is suitable for use in a thin, small wireless tag.

Solution to Problem

In order to attain the above object, an antenna in accordance with the present invention includes: a ground plate provided on a first plane; an antenna element at least part of which is provided on a second plane; and a capacitor that bridges the ground plate and a first end part of the antenna element, the first end part being opposite to a second end part that is a power feed-side end part. It should be noted here that the second plane is identical to the first plane or a plane parallel to the first plane.

According to the configuration, the antenna element is provided on a plane where the ground plate is provided or on a plane parallel to the ground plate. Therefore, it is possible to realize a thin antenna that can be mounted in or on a wireless tag. In addition, the capacitor makes it possible to cause the operating frequency band of the antenna to shift lower (i.e., possible to lower the resonant frequency) without increasing the total length of the antenna element. Accordingly, it is possible to realize an antenna configured to operate in a lower frequency band, without increasing its size. It should be noted that the antenna is a monopole antenna including a ground plate and an antenna element. Therefore, the antenna has a higher radiative power than a loop antenna that has an antenna element of the same length, and is also suitable for use in an active tag.

It should be noted that, in a case where part of the antenna element is provided on the second plane, that is, a part of the antenna element is provided on the second plane and the other part of the antenna element is not provided on the second plane, such other part may be provided on the first plane or may be provided on a third plane that is parallel to both the first plane and the second plane.

Advantageous Effects of Invention

As described above, an antenna in accordance with the present invention includes: a ground plate provided on a first plane; an antenna element at least part of which is provided on a plane that is identical to the first plane or on a second plane that is parallel to the first plane; and a capacitor that bridges the ground plate and a first end part of the antenna element, the first end part being opposite to a second end part that is a power feed-side end part. Therefore, according to the present invention, it is possible to realize an antenna configured to operate in a lower frequency band, without increasing its size. Furthermore, the antenna realized by the present invention is suitable for use in a wireless tag.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a top view illustrating a configuration of an antenna in accordance with Embodiment 1 of the present invention.

FIG. 2 is for explaining the effects of a shortening capacitor included in the antenna illustrated in FIG. 1. (a) of FIG. 2 is an equivalent circuit of the antenna. (b) of FIG. 2 illustrates a monopole antenna A1, an antenna A2 obtained by adding the shortening capacitor to the monopole antenna A1, and a monopole antenna A3 having the same resonant frequency as the antenna A2.

FIG. 3 is for explaining a capacitance of the shortening capacitor included in the antenna illustrated in FIG. 1. FIG. 3 illustrates (i) a monopole antenna B1 including an antenna element whose total length is λ/4 and (ii) a monopole antenna B2 which includes an antenna element whose total length is h and to which the shortening capacitor having a capacitance C has been added.

(a) of FIG. 4 is a top view illustrating a structure of a part, which includes a power feed point, of the antenna illustrated in FIG. 1. (b) of FIG. 4 is a top view of a modified example of (a) of FIG. 4.

(a) of FIG. 5 is a top view illustrating a structure of a part, which includes the antenna element, of the antenna illustrated in FIG. 1. (b) and (c) of FIG. 5 are each a top view of a modified example of (a) of FIG. 5.

FIG. 6 is a top view illustrating an example of the antenna in accordance with Embodiment 1 of the present invention.

FIG. 7 shows Smith charts obtained by plotting S-parameters (S11) of the antenna illustrated in FIG. 6. (a) of FIG. 7 is a Smith chart when no shortening capacitor is added. (b) of FIG. 7 is a Smith chart when a 2-pF shortening capacitor is added. (c) of FIG. 7 is Smith chart when a 3-pF shortening capacitor is added. (d) of FIG. 7 is a Smith chart when a 4-pF shortening capacitor is added.

FIG. 8 is a top view illustrating a configuration of an antenna in accordance with Embodiment 2 of the present invention.

FIG. 9 is for explaining that the antenna illustrated in FIG. 8 has resonant frequencies corresponding to the shapes of respective regions R1 and R2. (a) of FIG. 9 shows that the antenna illustrated in FIG. 8 has a resonant frequency corresponding to the resonant frequency of a particular dipole antenna. (b) of FIG. 9 shows that the antenna illustrated in FIG. 8 has a resonant frequency corresponding to the resonant frequency of a particular monopole antenna.

FIG. 10 is a perspective view illustrating a configuration of an antenna in accordance with Embodiment 3 of the present invention.

FIG. 11 is a perspective view illustrating a modified example of the antenna illustrated in FIG. 10.

FIG. 12 is a perspective view illustrating another modified example of the antenna illustrated in FIG. 10.

FIG. 13 is a graph showing the maximum electric field strength allowed for low power radio stations, which is specified in a law and a registration (Article 4 of the Radio Act and Article 6 of Regulations for Enforcement of the Radio Act) in Japan.

DESCRIPTION OF EMBODIMENTS
Embodiment 1

The following description will discuss Embodiment 1 (hereinafter referred to as “the present embodiment”) of the present invention with reference to the drawings.

(Configuration of Antenna)

The following description discusses, with reference to FIG. 1, a configuration of an antenna 1 in accordance with the present embodiment. FIG. 1 is a top view illustrating the configuration of the antenna 1 in accordance with the present embodiment.

As illustrated in FIG. 1, the antenna 1 is an inverted F antenna including a ground plate 11, an antenna element 12, and a short-circuit section 13. The ground plate 11, the antenna element 12 and the short-circuit section 13 are provided on an identical plane (hereinafter also referred to as an “antenna formation plane”) such that they do not overlap each other, and constitute a thin, planar antenna suitable for use in a wireless tag 2 (the outline of the wireless tag 2 is shown by a dotted line in FIG. 1).

The ground plate 11 is a planar (plate-like) conductor provided on the antenna formation plane. The ground plate 11 is also called a “planar ground”. In the antenna 1, the ground plate 11 serves to enhance an electromagnetic wave emitted from the antenna element by Miller effect. According to the present embodiment, the ground plate 11 is a rectangular piece of conductive foil. The ground plate 11 has, at its edge, a recess 11a and a protrusion 11b. Specifically, the recess 11a is in a position between an end (the right end in FIG. 1) and middle of the short side 11A. An end part 12A of the antenna element 12 (described later) is positioned in an area defined by the recess 11a. On the other hand, the protrusion 11b is at an end (the left end in FIG. 1) of the short side 11A. An end part 12B of the antenna element 12 (described later) faces the protrusion 11b.

The antenna element 12 is a linear (wire-shaped) or ribbon-shaped (shaped like a ribbon) conductor provided on the antenna formation plane. The antenna element is also called a “radiating element”. According to the present embodiment, the antenna element 12 is a bent ribbon-shaped piece of conductive foil. Specifically, the bent ribbon-shaped piece of conductive foil is constituted by (1) a first linear part 12a that extends from the end part 12A along the positive direction of a Y axis, (2) a second linear part 12b that extends, along the positive direction of an x axis, from an end of the linear part 12a which end is opposite to the end part 12A, (3) a third linear part 12c that extends, along the positive direction of the y axis, from an end of the second linear part 12b which end is opposite to the first linear part 12a, (4) a fourth linear part 12d that extends, along the negative direction of the x axis, from an end of the third linear part 12c which end is opposite to the second linear part 12b and (5) a fifth linear part 12e that extends, along the negative direction of the y axis, from an end of the fourth linear part 12d which end is opposite to the third linear part 12c. It should be noted here that the x axis and the y axis are parallel to the short side 11A and a long side 11B of the ground plate 11, respectively.

The end part 12A (in the present embodiment, the end part 12A is at an end of the first linear part 12a which end is opposite to the second linear part 12b) of the antenna element 12 is, as described earlier, positioned in the area defined by the recess 11a in the ground plate 11, and forms a power feed section with the recess 11a in the ground plate 11. In FIG. 1, a power feed point for the antenna element 12 and a power feed point for the ground plate 11 are represented as P and Q, respectively. The end part 12A of the antenna element 12 is hereinafter also referred to as a “power feed-side end part”.

The end part 12B (in the present embodiment, the end part 12B is at an end of the fifth linear part 12e which end is opposite to the fourth linear part 12d) of the antenna element 12 faces the protrusion 11b of the ground plate 11 as described earlier, and is connected to the protrusion 11b via a capacitor 14. The capacitor 14 serves to achieve a lower operating frequency band (lower resonant frequency) of the antenna 1 without changing the length of the antenna element 12 (this is described later). In other words, the capacitor 14 serves to shorten the antenna element 12 without changing the operating frequency band of the antenna 1. More specifically, the capacitor 14 serves to reduce the total length ρ of the antenna element 12 from λ/4 to λ/8 or less without changing the operating frequency band of the antenna 1. The capacitor 14 is hereinafter also referred to as a “shortening capacitor”.

The short-circuit section 13 is a linear or ribbon-shaped conductor provided on the antenna formation plane, and is arranged to short-circuit an intermediate portion 12C of the antenna element 12 and an edge of the ground plate 11. The short-circuit section 13 serves to match the input impedance of the antenna 1 to the output impedance of an IC chip 21 (described later). According to the present embodiment, the intermediate portion 12C is a portion between the second linear part 12b and the third linear part 12c, and the short-circuit section 13a is a ribbon-shaped piece of conductive foil provided along a normal to the short side 11A of the ground plate 11 extending from the intermediate portion 12C. With this configuration, a power feed line part (a part extending from the end part 12A to the intermediate portion 12C) of the antenna element 12 is positioned in a region defined by the ground plate 11, the short-circuit section 13 and a main part (a part extending from the intermediate portion 12C to the end part 12B) of the antenna element 12. It should be noted that the “intermediate” as in the intermediate portion 12C means that the intermediate portion 12C lies somewhere between the end part 12A and the end part 12B, and does not necessarily mean that it is in the midpoint between the end part 12A and the end part 12B.

The ground plate 11, the antenna element 12 and the short-circuit section 13 can be formed, for example, integrally on a PET (polyethylene terephthalate) film, which is a planar substrate, by printing with conductive silver paste. It should be evident that such a configuration makes it possible to realize an extremely-thin antenna 1 that is suitable for use in the wireless tag 2. Examples of the material for the planar substrate not only include PET but also include various dielectric materials such as glass epoxy and polyimide.

When the antenna 1 is mounted in or on the wireless tag 2, the antenna 1 is provided so that (i) the long side 11B of the ground plate 11 is parallel to a long side 2B (e.g., 85.6 mm) of the wireless tag 2 and (ii) the antenna element 12 and the short-circuit section 13 are positioned in a region defined by the short side 11A of the ground plate 11 and a short side 2A (e.g., 54.0 mm) of the wireless tag 2 (see FIG. 1). The reason why the antenna 1 can be placed like this is that the total length ρ of the antenna element 12 is reduced to λ/8 or less by the effect of the capacitor 14.

In fact, when the antenna 1 is to be operated at 315 MHz, absent the shortening capacitor 14, the total length ρ of the antenna element 12 should be approximately 25 cm (equivalent to λ/4). Therefore, it is difficult to place the antenna element 12 within the above-mentioned region regardless of how the antenna element 12 is bent. On the other hand, in a case where there is provided the shortening capacitor 14, the total length ρ of the antenna element 12 can be approximately 10 cm (equivalent to λ/10). Therefore, by bending the antenna element 12 as described earlier, it is possible to easily place the antenna element 12 within the above-mentioned region.

Furthermore, when the antenna 1 is mounted in or on the wireless tag 2, the IC chip 21 and a paper battery 22, which are to be mounted in or on the wireless tag 2 together with the antenna 1, may be arranged so as to overlap the power feed points P and Q and the ground plate 11, respectively (see FIG. 1). Since the IC chip 21 overlaps the power feed points P and Q, the IC chip 21 can be connected directly to the power feed points P and Q without a coaxial cable etc. Accordingly, high-frequency signals can be more efficiently exchanged between the antenna and the IC chip. Furthermore, since the paper battery 22 overlaps the ground plate 11, the paper battery 22 is prevented from overlapping the antenna element 12 which lies within the above-mentioned region. This makes it possible to prevent, for example, the following situations: (i) electric current induced in the paper battery 22 cancels an electric field around the antenna element 12 and thereby radiant intensity decreases and (ii) the paper battery 22 causes a distortion of an electromagnetic field around the antenna element 12 and thereby the radiation from the antenna element 12 becomes nonuniform. The battery mounted in or on the wireless tag 2 is not limited to the paper battery 22, and may be some other battery such as a button battery. Also in this case, the ground plate 11 may be placed so as to overlap the button battery.

The antenna 1 in accordance with the present embodiment is an inverted F antenna including the ground plate 11 and the antenna element 12 plus the short-circuit section 13. Note, however, that the antenna 1 is not limited to such. Specifically, the antenna 1 in accordance with the present embodiment can be an inverted L antenna which includes the ground plate 11 and the antenna element 12 but does not include the short-circuit section 13. Also in this case, the total length ρ of the antenna element 12 can be reduced by the effect of the shortening capacitor 14.

(Shortening Capacitor)

A main feature of the antenna 1 in accordance with the present embodiment resides in the shortening capacitor 14, which bridges the ground plate 11 and the end part 12B of the antenna element 12 which end part is opposite to the power feed-side end part. The shortening capacitor 14 makes it possible to reduce the total length ρ of the antenna element 12 without changing the operating frequency band of the antenna 1. In other words, it is possible to cause the operating frequency band of the antenna 1 to shift lower without changing the total length ρ of the antenna element 12. It should be noted here that, as for the antenna element 12 illustrated in FIG. 1, the total length ρ of the antenna element 12 is the sum of the lengths of the five linear parts 12a to 12e.

Effects of the shortening capacitor 14 are described in more detail with reference to FIG. 2. In the following description, a monopole antenna is taken as an example for convenience of description. Note, however, that the same applies to any kind of monopole-type antenna (i.e., antenna that includes a ground plate and an antenna element and operates on the same principle as a monopole antenna) such as an inverted F antenna and an inverted L antenna.

As has been well-known, a monopole antenna is equivalent to a series RLC resonant circuit illustrated in (a) of FIG. 2. In (a) of FIG. 2, R is radiation resistance, Le is effective inductance, and Ce is effective capacitance. The effective inductance Le and the effective capacitance Ce depend on the material and shape etc. of the antenna element. Impedance Z is given by Equation (1), and resonant frequency fo is given by Equation (2):

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